The fabrication and characterization of nano-scale tips is an important issue for scientists seeking maximum information from any of the various scanned probe microscopes. Nano-tips are required for well-defined studies of point contacts to metals or semiconductors as well as for manipulation and examination of atoms, molecules and small particles. Nano-tips are demanded for future multiprobe experiments where limits on probe-to-probe spacing are a direct function of the tip shape, needed as a source of coherent and bright electron beams for high-resolution electron microscopy, required for use as gas field ion sources (GFISs) for scanning ion microscopes (SIMs), and desired for applications involving high resolution AFM (atomic force microscope) and STM (scanning tunneling microscope) imaging and atom scale manipulation for pattern fabrication. In all of these cases, well defined, easily formed, clean and ultra-sharp tips will be advantageous. To our knowledge several techniques have been developed by other researchers to fabricate nano-tips: (1) the deposition technique, (2) the build-up technique, (3) the pseudo-stationary profile technique, (4) the field-surface-melting technique, and (5) the field-enhanced diffusion-growth technique. (6) Apex faceting has also been employed to try to shape nano-tips. Faceting can involve the deposition of additional metals or gas adsorbates following by an annealing procedure. (7) Proprietary process to fabricate a trimer-based or (8) field assisted gas etching.
There has been significant and recent interest in developing nanotips for gas field ion sources (GIFSs) for scanning ion microscopy (SIM) applications. The GFISs rely on principles of field ion microscopy (FIM), which was widely used and studied in the 1960s [E. W. Muller and T. T. Tsong. Field Ion Microscopy. Elsevier, New York, 1969]. In FIM, a sharp tip is placed in a vacuum chamber while high positive bias voltage is applied. When an imaging gas (usually He) is introduced into the chamber, an FIM pattern is observed on a fluorescent screen. A schematic diagram of a typical FIM set up is shown in FIG. 1. Each spot on the fluorescent screen (see FIG. 2) represents an individual ion beam generated by a single surface atom. In SIM, a single surface atomic site is selected by an aperture to be an imaging beam of the microscope. Atomically defined tips are desired for SIM operation to achieve high ion current and brightness since imaging gas is shared among fewer atomic sites.
The GFISs are well suited for imaging as well as for nano-machining application using non-staining ion He+ and Ne+ beams, and are considered for replacement of LMIS (liquid metal ion sources) in FIB (Focused Ion Beam) microscopes in order to perform imaging or nano machining. The LMIS is a very sharp metal tip with a Ga metal coating. At high voltage, Ga ions are emitted. LMISs are currently found in most FIB microscopes and can produce a beam of approximately 5 nm width. Moreover, conventional LMIS-based ion microscopes are primarily used for machining but have the undesired trait of “staining” samples under study with residual gallium. As a result, dual beam electron imaging and ion milling machines are required and most commonly used. A helium-based GFIS microscope will leave little to no residual helium on the sample. A GFIS is also based upon a sharp metal tip but atoms arriving from the surrounding gas phase are ionized and emitted. A microscope based upon GFIS can produce a beam of approximately 1 Angstrom width. An ion beam of neon or argon could be used for nano machining without undesirable staining effect.
There are also a number of advantages SIM offers over conventional SEM including (i) better depth of focus due to a highly collimated ion beam, (ii) better ultimate resolution due to shorter wavelength and lower spherical and chromatic aberrations, and (iii) superior element and surface sensitivity. However, the development of a scanning ion microscope has been hampered by the lack of an acceptable gas field ion source (GFIS). In order to achieve the potential of the SIM, some ideal characteristics of GFISs for SIM need to be achieved: 1) the ion sources must be easy to fabricate, 2) the sources should be readily rebuilt without removal from the microscope, 3) ideally, each rebuild should result in a functionally identical apex structure such that emission properties, including the emission axis, remain unchanged, thereby eliminating the need for substantial gun alignment, 4) the source must support large ion currents and be stable for extended periods of time, and 5) in order to improve performance of the microscope, it is also desirable that angular current intensity of the ion beam be as large as possible to maximize probe current.
The field assisted chemical etching method has been described and involves the adsorption of a gas species, typically nitrogen, on the surface, e.g. tungsten. The adsorption of molecular nitrogen on tungsten surfaces has been thoroughly investigated [T. Tamura and T. Hamamura, Surf. Sci. 95, L293, 1980]. It has been found that several adsorption states are formed; among these is the “strong-bond” state. This state arises from the dissociation of N2 on the tungsten surface followed by diffusion into the top atomic layer of W. This causes a protrusion of W atoms, which results in a weak surface structure. Early FIM studies of nitrogen gas on tungsten tips found that the nitrogen reaction only occurs in low field regions, where it can penetrate the ionizing barrier. K. D. Rendulic and Z. Knor [Surf. Sci. 7, 205, 1967] also showed that when a W tip was exposed to nitrogen gas, holes developed on the (111) and (001) planes, resulting from the removal of W atoms. This corrosive reaction of nitrogen was explained as follows: the protrusion of W atom from the metal surface, caused by the adsorption of N2, leads to an enhanced electric field, which becomes adequate to ionize and then evaporate the protruding W atoms. [G. Cranstoun, J. Anderson, Surface Science v 35, p 319. 1973].
U.S. Pat. No. 7,431,856 covers the fabrication of nanotips using a field assisted etching process to create single atom tips. Once this process was made public, other gases have also now been used to create nanotips through the chemical assisted field induced etching mechanism. Oxygen has been shown to be viable etchant gas for the creation of nanotips and a constant voltage etching method has been described for the creation of nanoprotrutions [Y. Sugiura et al. e-J. Surf. Sci. Nanotech. Vol. 9 (2011) 344-347]. Water has also been found to aggressively etch tungsten tips and has been used with a constant voltage etching method for the preparation of tips for field emission studies [Jo Onoda, Seigi Mizuno. Applied Surface Science 257 (2011) 8427].
U.S. Pat. No. 7,431,856, the contents of which are herein incorporated by reference, describes a method of fabricating SATs (Single Atom Tips) using field-assisted ion etching. There remain, however, some outstanding issues. In particular, it would be desirable to develop a method to prepare SATs with different global shapes. The global shape is an important parameter since it controls beam divergence angle of charged particle beams as well as modifies tip-sample interactions in scan probe applications. For instance, (i) in the SIM application and ion beam generation, the ability to increase the angular current intensity (brightness), ideally using a SAT on a broad base, is important in order to improve the microscope performance. (ii) In point-projection electron holography a divergent electron beam, such as obtained from a tall, narrow tip, is desirable to improve resolution. (iii) In scan probe microscopy (e.g. AFM) various shaped tips provide different advantages via shape dependent tip-sample interactions.
In an operating SIM, SAT ion sources fail over time. It would also be desirable to prepare and subsequently rebuild SATs in situ with a consistent tip position/beam orientation so as to maintain the existing alignment. This ensures identical emission character with respect to peak angular emission. Such source emission angle consistency eliminates the requirement of gun realignment. The development of an acceptable SAT for operation with gas supplies other than helium, including neon, would also be useful.
Trimer tips have found a commercial application and offers reasonable structural stability and robustness. However, W (111) trimer-based GFISs may require elaborate alignment procedure to select one of the three ion beams. Moreover, all three atomic sites at the tip apex have to share helium gas supply, limiting the potential current. As well, poor stability while creating neon ion beams, crucial for performing ion milling (machining) at the nanometer scale, has been observed. On the other hand, single atom tips (SATs) allow all available imaging gas atoms to be ionized at the single apex atom offering the largest ion current per atom. Also, the nitrogen assisted etching and evaporation process provides that every subsequent tip rebuilding process results in the SAT with the exact same apex atom lateral placement and orientation and can prepare stable beams of neon for imaging and nanomachining.